Epoxy resins are utilised in many applications, like coating materials, electrical insulating materials, adhesives, structural reinforcements because the cured material has excellent mechanical, electrical, thermal properties, good chemical resistance and adhesion properties. Epoxy resin systems are generally supplied as two components. One is the resin and the other the hardener. Both must be mixed before use. The mixed components have a limited shelf life and cure. They can not be stored.
Properties of the cured materials can be changed through the formulation of the epoxy resin or of the hardener. For what regards epoxy resins a large variety of resins containing liquid epoxy resins based on reaction product of bisphenol A with epichlorohydrine are formulated with reactive diluents, bisphenol F resins, epoxy Novolaks, plasticizers, toughening agents, fillers to impart specific properties in terms of thermal resistance, chemical resistance, mechanical and electrical properties, flame resistance, thermal conductivity, etc.
The cured mixtures must be tested for complete curing via measurement of glass transition temperature (Tg) or heat distortion temperature (HDT) via differential scanning calorimetry (DSC), thermo mechanical analysis (TMA), dynamic mechanical analysis (DMA), dielectric properties, mechanical properties which are laborious methods, mainly if curing is done in field conditions, like curing of an epoxy adhesive for tube liners.
As curing agents that are effective at ambient temperature or moderate temperatures the state of the art are amines with more than one primary or secondary amine group. Curing of the epoxy resins can also be done via a homopolymerization. In this case catalysts e.g. based on tertiary aliphatic amines, salified amines, imidazoles, are used.
In EP 329 456 aromatic polynuclear aromatic diamines were claimed as hardeners for epoxy resins. Curing is monitored via Tg.
Various substituted 1,3-diaminopropanes can be used as hardeners for epoxy resins like described in GB 1 571 457. Tg is used to prove the curing.
Because polyamines are difficult to handle, have unpleasant odors and sometimes are hazardous substances, sometimes derivatives of the amines are used as hardeners. DE 23 36 353 describes the use of the reaction product of aliphatic diamines with methyl caprolactame as aminic hardener. Mechanical values were used to characterise the cured material. EP 1 731 545 describes the use of the reaction product between an epoxy resin and methylimidazol as aminic hardener. EP 507 735 describes the use of the reaction product of cycloaliphatic or heterocycloaliphatic diamines with polyfunctional epoxy resins as aminic hardeners. Mechanical properties and Tg were use to control curing. Epoxy formulations containing latent catalysts derived from salification of tertiary amines are another possible class of usable products. U.S. Pat. No. 4,449,246 describes one with particular reference to powder coating.
Organic colourants known can be of low or high molecular weight. Polymeric colourants are known, and were described e.g. in U.S. Pat. No. 4,284,729. They are based on a polymeric constituent derived from polyalkylene oxides or its copolymers reacted with dyestuff radicals. These polymeric colourants are reported to have reactive groups on the end of the molecules to be able to covalently bond to the polymeric matrix to be dyed.
U.S. Pat. No. 3,157,633 describes several methods to produce them and their use as fugitive dyes in several applications related to textiles. These reactive polymeric dyes are used to colour polymeric material. The dyes have the capability to bond to the matrix.
Several cases are reported of colour changes of these dyes as consequences of variations of pH. This is a main drawback in their use. To overcome this, specific structures were designed as in U.S. Pat. No. 7,019,101.
Several applications can benefit from a simple way to detect the curing of epoxy resins. Among them pipe relining represents a relevant case due to the difficulties in assessing the proper cure of pipe laying below the level of the floor/street.
U.S. Pat. No. 4,009,063 describes the process of pipe rehabilitation through the insertion of a liner which is impregnated with thermosetting resins, mainly polyester or epoxy based resins. Curing starts by thermal, UV or other activation. Still the most preferred is the thermal activation for its simplicity. Many products based on different epoxy system compositions are available on the market for repairing damaged pipes. They cover different needs in terms of reactivity and final properties. U.S. Pat. No. 5,017,258 describes a typical epoxy system used in this field to balance a long pot-life with a fast curing at moderate temperature.
From U.S. Pat. No. 3,030,329; WO 94/12988, WO 2005/030853 and JP 56036540 compositions are known that use triaryl compound-based dyes to indicate the mixing and gel-state of epoxy resins.